Helical agitators are frequently used for mixing liquids and solids. The object of the agitator is, with the lowest possible energy expenditure, to achieve the shortest possible mixing times up to homogenization of the material to be mixed. Many data on the mixing behavior of helical systems may be found in the literature [Tatterson, G. B.; Fluid mixing and gas dispersion in agitated tanks; McGraw-Hill Inc.; 1991; pp. 325ff]. Helical agitators are popular mixer geometries for homogenizing relatively high viscosity products.
The demands which are made of the mixing behavior by chemical and other production processes are constantly increasing, since decreased mixing time at the same energy input lead to reduced overall costs in production.
To increase the space-time yield in production processes, it is, furthermore, of interest to carry out as many process steps as possible in one mixing apparatus. Apparatuses having agitators must therefore be able to homogenize materials for mixing with low mixing times despite great viscosity changes. This mixing task can be fulfilled only inadequately using conventional helical agitator arrangements having one shaft. In the literature there have already been frequent descriptions that mixing times increase many times in the medium-viscosity flow range [Tatterson, G. B.; Fluid mixing and gas dispersion in agitated tanks; McGraw-Hill Inc.; 1991; page 381]. This is a great disadvantage for processes having viscosity changes which require a short mixing time at any time point for means of quality and efficiency.
DE 10248333 A1 describes mixers in which the mixing tools sweep mutually, and the container wall completely, in order to achieve self-cleaning of the mixer as completely as possible, similarly to the case with closely-meshing twin-shaft extruders. In this publication a description is likewise given that the mixing times of such self-cleaning systems are significantly decreased compared with conventional helical agitators.
The above-described mixer system, however, has the disadvantage that it can only be produced at very high cost, because the mixing tools to achieve the self cleaning, similarly to gearwheels, must be fabricated having a precise geometry matched to one another, and be driven by an exact synchromesh gear.
FR-A 94 02618 describes a further mixer having one central agitator and one eccentric agitator, the eccentric agitator fitting into a recess of the central agitator. This mixer, however, has the disadvantages that it does not generate targeted mixing in the direction parallel to the shafts, since the agitators have no axial transport action, and that the drive moment at the central shaft always passes through a high peak value when the blade of the central agitator having its recess sweeps past the blade of the eccentric agitator. This leads firstly to increased costs for the drive energy of the mixer; secondly, the high peak torque in the mutual passage of the agitators must be compensated for by a stronger construction of the agitators and of the drive, which in turn makes the entire structure more expensive.
GB 2076675A describes trough mixers having helical mixer geometry. They are widely used for mixing bulk goods or pasty media. These horizontally arranged mixers have the disadvantage that their housing must be formed in complex and thus expensive geometric shapes. In addition, it is a disadvantage that the mixers must be operated with their housing only in horizontal arrangement.
The object was therefore to provide a mixer having a cylindrical housing which has good axial and radial mixing of the material for mixing, retaining good mixing time even during large viscosity changes in the process and with low costs both for fabrication of the mixing tools and of the drive and also for the energy employed for mixing.